crypto/hmac: wrap ErrUnsupported returned by Clone

Updates #69521

Change-Id: I6a6a4656403b9d35d5e4641b5c5c4975f3fa0e43
Reviewed-on: https://go-review.googlesource.com/c/go/+/675555
Reviewed-by: Austin Clements <austin@google.com>
Auto-Submit: Filippo Valsorda <filippo@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Roland Shoemaker <roland@golang.org>

src/crypto/hmac/hmac_test.go

@@ -11,6 +11,7 @@ import (
 	"crypto/sha1"
 	"crypto/sha256"
 	"crypto/sha512"
+	"errors"
 	"fmt"
 	"hash"
 	"testing"
@@ -583,6 +584,18 @@ func TestHMAC(t *testing.T) {
 	}
 }
 
+func TestNoClone(t *testing.T) {
+	h := New(func() hash.Hash { return justHash{sha256.New()} }, []byte("key"))
+	if _, ok := h.(hash.Cloner); !ok {
+		t.Skip("no Cloner support")
+	}
+	h.Write([]byte("test"))
+	_, err := h.(hash.Cloner).Clone()
+	if !errors.Is(err, errors.ErrUnsupported) {
+		t.Errorf("Clone() = %v, want ErrUnsupported", err)
+	}
+}
+
 func TestNonUniqueHash(t *testing.T) {
 	if boring.Enabled {
 		t.Skip("hash.Hash provided by boringcrypto are not comparable")

src/crypto/internal/fips140/hmac/hmac.go

@@ -130,26 +130,36 @@ func (h *HMAC) Reset() {
 	h.marshaled = true
 }
 
+type errCloneUnsupported struct{}
+
+func (e errCloneUnsupported) Error() string {
+	return "crypto/hmac: hash does not support hash.Cloner"
+}
+
+func (e errCloneUnsupported) Unwrap() error {
+	return errors.ErrUnsupported
+}
+
 // Clone implements [hash.Cloner] if the underlying hash does.
-// Otherwise, it returns [errors.ErrUnsupported].
+// Otherwise, it returns an error wrapping [errors.ErrUnsupported].
 func (h *HMAC) Clone() (hash.Cloner, error) {
 	r := *h
 	ic, ok := h.inner.(hash.Cloner)
 	if !ok {
-		return nil, errors.ErrUnsupported
+		return nil, errCloneUnsupported{}
 	}
 	oc, ok := h.outer.(hash.Cloner)
 	if !ok {
-		return nil, errors.ErrUnsupported
+		return nil, errCloneUnsupported{}
 	}
 	var err error
 	r.inner, err = ic.Clone()
 	if err != nil {
-		return nil, errors.ErrUnsupported
+		return nil, errCloneUnsupported{}
 	}
 	r.outer, err = oc.Clone()
 	if err != nil {
-		return nil, errors.ErrUnsupported
+		return nil, errCloneUnsupported{}
 	}
 	return &r, nil
 }

src/hash/hash.go

@@ -57,13 +57,14 @@ type Hash64 interface {
 	Sum64() uint64
 }
 
-// A Cloner is a hash function whose state can be cloned.
+// A Cloner is a hash function whose state can be cloned, returning a value with
+// equivalent and independent state.
 //
 // All [Hash] implementations in the standard library implement this interface,
 // unless GOFIPS140=v1.0.0 is set.
 //
-// If a hash can only determine at runtime if it can be cloned,
+// If a hash can only determine at runtime if it can be cloned (e.g. if it wraps
-// (e.g., if it wraps another hash), it may return [errors.ErrUnsupported].
+// another hash), it may return an error wrapping [errors.ErrUnsupported].
 type Cloner interface {
 	Hash
 	Clone() (Cloner, error)